DE 40 02 843 describes a breathing mask comprising means for judging the efficiency of the filter and for determining the moment when it is necessary to change the filter. The mask comprises for this purpose two transducers which are placed on either side of the filter to be monitored. The cited document proposes to form a differential signal from the signals provided by the two transducers, which differential signal is used for giving a warning signal when it is necessary to replace the filter. This differential signal is sent to a monitor. At the same time, the signal supplied by the sensor present downstream of the filter is compared with a limit value such that an alarm signal is triggered when this signal of the downstream sensor reaches the limit value.